If you've wanted a comprehensive, balanced set of rules for psionics in 5th edition, I highly encourage you to check this release out. Especially if you weren't excited about psionics not being a separate thing in the recent UA, or if you're trying to figure out why psionics is cool - this is a book that will sell you on why psionics is cool, and why its system of psionics is cool.
It balances psionics by making you specialize in them (unlike the Mystic), which leads to very interesting things like psions which are good at changing the reality around them, compared to empaths which manipulate emotions and feeling. This ends up feeling much more natural and gives a lot more options through depth, rather than simply by giving every option possible (which was a common criticism of psionics). It's a similar approach to psionics as D&D has always taken to magic: wizards specialize in types of spells, such as "controlling" or "blasting," clerics can specialize in healing or so on - why shouldn't the same be true for psionics?
Lastly, I wouldn't let the price tag fool you - this is a hefty release, comparable to one of the most popular previous releases by the author: Adventurer's Almanac, which is one hundred and ninety three pages of dense and well-balanced mechanics.
In the Psion's Primer you get two full classes with three subclasses each, three subclasses which augment existing classes (Fighter, Rogue, and Monk), and the full psionics system - that's just by chapter 1! Chapter 2 includes mechanics for psionic races, including several that are setting-agnostic, like Thri-kreen. There are quite a few psionic feats and items, and finally a little over 50 pages of Psionic NPCs - perfect for DMs who want to introduce enemies like the Inspired, true-psionic Mind Flayers, Duergar, or even interesting bits from previous editions like the Psion Killer.
It's worth the money and time - definitely check out the preview.
[5 of 5 Stars!]